Conventional concrete, as a construction material, suffers from a number of inherent deficiencies. The primary drawbacks relate to its lack of ductility, low tensile strength, and a tendency to undergo significant shrinkage during curing. The brittle nature of concrete has had a direct effect on the specifications and guidelines used in the design of concrete structures. Since the ultimate goal in the design of any structure is generally safety, there are a number of precautions that needed to be taken when formulating the design approaches used today. Chief among these is the avoidance of brittle failure modes.
If a structure were to fail in service, people inside that structure would be at great risk if they had insufficient warning to vacate the premises before collapse. If such a failure occurred instantaneously, as in brittle behavior, there would be no warning. Alternatively, if there was a large amount of deformation, movement and noise produced by the structure before failure (ductile), people would have time to get out. Current design codes recognize this dilemma and base their criteria around ductile failures. The question becomes how to force a brittle material (concrete) to fail in a ductile manner.
The behavior of typical concrete beam to which a uniform load is added is well known. When the load is applied, the beam deflects. This causes a shortening of the upper surface of the beam, resulting in compressive stresses in this region of the member as the material of the beam (i.e. concrete) tries to resist the change in shape. The bottom surface, on the other hand, is lengthened or stretched, resulting in an induced tensile stress as the concrete tries to resist elongation.
Concrete is relatively strong in compression but very weak in tension. If the beam were to be made entirely from concrete, it would fail at the bottom surface under a very low load, possibly even its own weight, and that failure would be very brittle in nature. Thus, something must be done to the lower portion of the beam to prevent the tensile stresses from failing the concrete.
This logic is the foundation for conventional reinforced concrete beam design. Generally, reinforcing bars are placed within the concrete beam, near the bottom, to carry tensile loads and alleviate the tensile stresses otherwise applied to the concrete. Steel, being much stronger than concrete in tension, is well suited for this application. In addition, steel fails in a very ductile manner, with very large amounts of elongation before failure. If this occurs within the concrete, a great deal of deformation and noise is generated, thus providing the warning necessary to save lives.
The basic theory behind conventional reinforced concrete beam design is well known. Essentially, steel reinforcement is placed near the bottom of the beam and is used to carry the tensile stresses while the concrete at the top of the beam carries the compressive stresses. To avoid failure of this concrete in compression, the steel is actually under-designed so that it will fail first. Thus, the concrete never reaches its ultimate capacity.
Furthermore, the concrete in the bottom portion of the beam is not even considered in the design since its strength is very low in tension, relative to the steel. Its job is simply to protect the steel from the surrounding environment by acting as a barrier to deleterious substances (e.g. seawater). Seawater will not significantly affect the concrete itself but can cause corrosion of the steel reinforcement, resulting in overall degradation of the structure. The effectiveness of this approach depends upon the inherent permeability of the concrete, which is directly dependant upon the presence and size of cracks. These cracks can and do occur because of such issues as shrinkage, overloading, fatigue loading, impact, and other durability mechanisms.
One method of improving both of these drawbacks (brittle failure mode, high permeability) is to provide reinforcement of the concrete matrix at a smaller scale than the steel bars. This is often done through the use of short fibers mixed into the concrete during batching. Fibers have the ability to improve durability by resisting crack opening and provide strength after initial cracking, thus improving the ductility of the concrete.
Permeability can also be improved by altering the concrete microstructure to produce a denser, less porous, arrangement of components. The most common approach to achieving this goal is the inclusion of a pozzolanic material in the concrete mix design.
Concrete is well known to be made up of two primary components; stone and sand aggregates surrounded by a hydrated cement paste (hcp) matrix. It is the latter which acts as the glue that binds the aggregates together. It is also the hcp that is the dominant factor when it comes to permeability, since the aggregates typically used in concrete tend to be far less permeable than the surrounding matrix.
Examining the hcp matrix reveals that there are two primary building blocks that make up its microstructure; calcium-silicate-hydrate (C—S—H) and calcium hydroxide. The C—S—H takes the form of very small crystals packed closely together to form a very dense structure. The calcium hydroxide, on the other hand, forms much larger, layered, plate-like crystals. These crystals do not pack well and tend to exhibit weakness between layers due to poor bonding. Ultimately, it is the calcium hydroxide that represents the weak link in both strength and permeability of hcp.
Pozzolanic materials are alumino-siliceous materials which reacts with calcium hydroxide in the presence of water to form compounds possessing cementitious properties at room temperature, producing calcium-silicate-hydrate (C—S—H). The end result is a significant reduction in porosity and permeability, accompanied by a corresponding increase in strength. Common pozzolans in use today include fly ash, silica fume, blast furnace slag, and high reactivity metakaolin.
Typical effects of commonly used pozzolanic materials on the amount of calcium hydroxide in concrete are shown in FIG. 1. The effect of this reduction in calcium hydroxide on permeability can be seen in FIG. 2, which shows the relative amounts of chlorides penetrating into different concretes after prolonged exposed to seawater.
Among the large number of clay types available, either natural or man-made, the polymerization of Montmorillonite (M-clay) has been the most actively studied. As defined herein and known in the art “clay” is a term used to describe a group of hydrous aluminium phyllosilicates minerals that are generally less than 2 μm in diameter that consist of a variety of phyllosilicate minerals rich in silicon and aluminium oxides and hydroxides which include variable amounts of structural water. There are three or four main groups of clays: kaolinite, montmorillonite-smectite, illite, and chlorite (chlorite is not always considered a part of the clays and is sometimes classified as a separate group, within the phyllosilicates). There are about thirty different types of “pure” clays in these categories but most “natural” clays are mixtures of these different types, along with other weathered minerals. Montmorillonite has a chemical formula of (Na,Ca)0.33(Al,Mg)2Si4O10(OH)2.nH2O.